You need to install the Xfree86 port, (or from a mirror site near you).
To run Eco Lab, you first need to start the X-server, using the command line XDarwinStartup [-quartz] (the -quartz option is needed to run X under Aqua). You will then be asked whether you want to run the server rootless or not. I advise against running rootless initially until you're more comfortable running X. Set the DISPLAY variable in your shell to ``unix:0'' -- localhost is not necessarily mapped to 127.0.0.1 on OSX!. Then start a window manager. You can either use the antiquated twm manager by typing twm& on the command line, or I have it good authority that the OroborOSX window manager is the way to go!
Performance of Xwindows under Aqua is reportedly not good. If you
don't need any other Aqua applications open, you can log off the Aqua
desktop, and log in with the username ``>console'' (which first needs
to be created). The gives a basic shell window, from which you can
launch Xwindows.